1. Field of the invention
The invention concerns an optical time-division multiplex broadband multiservice communication system operating asynchronously and enabling the implementation of integrated optical communication networks in which communication signals are transmitted and switched optically without any intermediate conversion of the transmitted signals.
2. Description of the prior art
The ATM (Asynchronous Transfer Mode) and STM (Synchronous Transfer Mode) information transfer techniques are well known and existing BISDN (Broadband Integrated Services Digital Networks) currently use one of these techniques.
When the STM technique is used the communicating transmitter and receiver are connected for the duration of the call by a circuit comprising one or more time-division buses made up of a succession of cyclic time-division multiplexes which usually have a frame structure. Time-division switching centers carry out the necessary space-division and time-division switching of the various multiplexes used by the circuit according to a cyclic procedure using control data stored for the duration of the call in control devices of the switches in each center. The information to be transmitted is formatted at the source into fixed format samples transmitted successively on the time division buses assigned to the call.
Many synchronous time-division switching devices have been described, in particular "controlled propagation" time-division switches using delay lines to rearrange the signals in time. French patent 2 119 152 of 22 Dec. 1970 describes switches of this kind and indicates that the delay lines may be implemented by means of optical fibers. An experimental optical propagation time-division switch has been produced by ATT Bell-Labs and is described in the article by R. A. Thomson and P. P. Giordano "Experimental Photonic Time-Slot Interchanger Using Optical Fibers as Re-Entrant Delay Line Memories", OFCC, 02/86. This switch uses optical fibers and electrically controlled directional couplers.
The ATM technique combines and adapts the time-division multiplex transmission and packet transmission techniques: the communicating transmitters and receivers are connected for the duration of the call by a virtual circuit which is allocated a physical path made up of a succession of time-division multiplex lines and switching centers. The time-division resources are assigned statistically from a pool of sources to which no specific bus is allocated on the lines. A source or a receiver may be involved in a multiplicity of different simultaneous calls to different destinations.
The information to be transmitted is formatted at the source into fixed format cells each comprising identification and routing information; the cells are transmitted acyclically on time-division multiplex lines which do not have a frame structure, according to the availability of non-dedicated transmission channels.
The switching of the cells is therefore similar to time-division switching, involving space-division switching and rearrangement in time. As the multiplexing mode is a statistical one, however, the time rearrangement resources are themselves used statistically and are therefore managed like queues. To switch a cell the identification and routing information it contains must first be analyzed.
ATM switching systems have been described, for example in French patent 2 538 976 of 29 Dec. 1982, covering a synchronous fixed length packet switching system. These known systems use optical transmission means to connect the users and to interconnect the switching centers and electronic means to switch the signals. Opto-electrical and electro-optical conversion of the signals are therefore carried out at the boundaries of the transmission means and the switching means, and the signals are demodulated in the switching means in order to consult the routing information. These conversion and analysis means complicate the structure of the system, compromise its reliability and increase its cost.
New communication techniques based on optical spectral multiplexing are known: optical transmitters and receivers are connected to a passive propagation medium which divides equally between the receivers the energy produced by each transmitter. The transmitters and the receivers can be tuned to a multiplicity of optical frequencies each of which constitutes a transmission channel for one communication direction. There are three main operating modes:
each transmitter is locked permanently to an allocated frequency, the call being set up by tuning the communicating receiver to said frequency, or
each receiver is locked permanently to an allocated frequency, the call being set up by tuning the communicating transmitter to said frequency, or
the frequency is allocated to a call when it is set up, the communicating transmitter and receiver being tuned to said frequency.
Frequency-domain switching systems of this kind have been described in the following patents:
EP 77 292 published 20 Apr. 1983 covering a communication network using monomode fibers as transmission means for optical buses multiplexed in the optical carrier frequency domain, PA1 FR 2 586 874 of 29 Aug. 1985 covering an optical fiber telecommunication device, and PA1 FR 2 593 654 of 28 Jan. 1986 covering a coherent photonic telecommunications device. PA1 the N input buffers and the N output buffers are electronic memories, PA1 the former are each electrically connected by an input device to the receiver of an optical input line and by an output device to a wavelength tunable laser, PA1 the latter are each electrically connected by an input device to an opto-electronic receiver tuned to a particular optical wavelength and by an output device to the transmitter of an optical output line, PA1 the N tunable lasers, each connected to one input buffer memory, and the N wavelength tuned opto-electronic receivers, each connected to one output buffer memory, are optically connected to an N.times.N passive optical star coupler, PA1 a packet is transferred between an input buffer memory and an output buffer memory by tuning the laser connected to the output of the input buffer memory to the wavelength to which is tuned the opto-electronic receiver connected to the input of the output buffer memory which is the destination for the duration of packet transmission. PA1 the signals to be transmitted undergo various conversions during the switching operations, including opto-electrical and electro-optical conversion (demodulation and remodulation of the optical signals), deserialization and serialization of the electrical signals, PA1 routing the packets requires use of routing information included in the packets themselves and therefore demodulation of the carrier signal, PA1 the optical communication system including the transmitters, distributor and receivers is confined to a restricted space, and PA1 the capacity of the switching means is limited by the number of wavelengths that can be discriminated, each wavelength being assigned to one receiver. PA1 at least one optical control receiver tuned to an optical frequency from a first group of optical frequencies of a first set of optical frequencies; PA1 an optical transmission receiver which can be tuned to any optical frequency of a second group of optical frequencies of said first set of optical frequencies; PA1 optical frequency tuning means for said optical transmission receiver; PA1 and means for temporal synchronization of the received information; PA1 an optical transmission transmitter that can be tuned to any optical frequency of said second group of optical frequencies; PA1 optical frequency tuning means for said optical transmitter; PA1 and temporal synchronization and phase alignment means for the information to be transmitted; PA1 an optical spectro-time-division switching network; PA1 a control processor unit; PA1 and a clock unit; PA1 at least one control transmitter circuit having an optical control transmitter tuned to an optical frequency of said first group of optical frequencies; PA1 at least one signalling transmitter having an optical transmission transmitter that can be tuned to any optical frequency of said second group of optical frequencies; PA1 at least one control receiver circuit and at least one signalling receiver each having at least one optical control receiver tuned to an optical frequency of said first group of optical frequencies and an optical transmission receiver which can be tuned to any optical frequency of said second group of optical frequencies; PA1 a control bus; PA1 at least one optical control transmitter line, each of which is connected optically to the optical control transmitter of a control transmitter circuit; PA1 at least one optical control receiver line, each of which is connected optically to the optical control and transmission receivers of a control receiver circuit; PA1 and at least one optical signalling transmitter line, each of which is connected optically to the optical transmitter of a signalling transmitter; PA1 at least one optical signalling receiver line, each of which is connected optically to the optical control and transmission receivers of a signalling receiver; PA1 an optical line scanner connected optically to the optical user transmitter lines and to the optical signalling transmitter lines, said optical transmitter lines each carrying a transmit spectro-time-division multiplex comprising time slots each carrying an optical information sample; PA1 an optical command distributor; PA1 a control circuit connected optically: PA1 n controlled propagation optical time-division switching units each connected optically to an output of the optical line scanner and to an output of the optical command distributor; PA1 an optical distributor connected optically to the output of the time-division switching units and to the optical control transmitter lines and having outputs each connected optically to the input of an optical filter; PA1 s optical wavelength filters each connected optically to the output of an optical user receiver line, signalling receiver line, control receiver line; PA1 a filter control circuit connected to the input of the control bus and to the output of a control input of each optical wavelength filter. PA1 a line scanner connected optically to the optical user transmitter lines and to the optical signalling transmitter lines, said optical transmitter lines each carrying a transmit spectro-time-division multiplex comprising time slots each carrying an optical information sample; PA1 an optical command distributor; PA1 a control circuit connected optically to the optical line scanner by a first optical line carrying an output label multiplex comprising as many time slots as there are transmitter lines; to the optical command distributor by a second optical line carrying a control label multiplex having the same number of time slots as said output label multiplex, each time slot of the control label multiplex being in phase with a same ranked time slot of said output label multiplex and carrying a command; and to the clock unit by the first synchronization line; and further connected to the control processor unit by the command bus; PA1 controlled propagation optical multiple time-division switching units having: PA1 an optical distributor connected optically to the output of the delay unit output circuit and to the optical control transmitter lines and having outputs each connected optically to the input of an optical filter; PA1 s optical wavelength filters each connected optically at its output to one of the optical user receiver lines, signalling receiver lines, control receiver lines; PA1 a filter control circuit connected by its input to the control bus and by its output to a control input of each optical filter. PA1 the optical carrier frequency from a first group Fa1 is used as a reference for the optical frequency-domain encoding systems of the terminals, PA1 the frame code is used as a time reference for temporal synchronization of the terminals, and PA1 the other time slots are used to transmit terminal control messages, PA1 a output label multiplex OLM, and PA1 a control label multiplex CLM; the multiplexes OLM and CLM are frame structured time-division multiplexes having a period equal to the duration of a time slot of the communication multiplexes and a number of time slots per frame equal at least to the total number of lines UEL and SEL connected to the spectro-time-division switching network SSTSN.
These systems do not operate in spectro-time-division mode. The optical frequencies used to convey the call are, in the best case, mobilized for the duration of the call.
Multiservice operation, whereby multiple calls to different destinations can be set up from the same terminal, is possible only if the terminals are each equipped with a multiplicity of transmit and receive devices. Each call set up mobilizes one or two optical frequencies.
From what is known of ATM systems, optical systems using frequency-domain multiplexing only would seem not to be well suited to multiservice operation.
Finally, use of a wavelength switching optoelectronic device instead of an electronic space-division switch to carry out the switching function between the input buffer memories (FIFO) and output buffer memories (Elastic Buffer) of a high-speed packet switch with N optical input lines and N optical output lines has been described in the article by M. S. Goodman et al of Bell Communications Research "Demonstration of Fast Wavelength Tuning For a High Performance Packet Switch", Conferences on Optical Communication, 11-15/09/88--Brighton UK, pages 255 through 258, IEE.
In this fast packet switch:
In this fast packet switch:
An object of the invention is to provide a broadband multiservice optical communication system in which signals are transmitted and switched end to end by optical means only without requiring successive multiple opto-electrical and electro-optical conversion or modulation and demodulation of said signals.
Another object of the invention is to provide a multiservice optical communication system in which a terminal can by means of the same optical transmitter and/or the same optical receiver set up a multiplicity of simultaneous calls having different destinations or sources.
Another object of the invention is to provide a multiservice optical communication system in which the use of optical wavelengths or frequencies is optimized to increase the system capacity in terms of terminal connections.
Another object of the invention is an asynchronous time-division multiplex optical communication system which does not require any analysis of the information transferred other than what is directly allowed by the optical and opto-electronic means alone without previous demodulation or conversion of the optical communication signals.
Another object of the invention is to space-division and time-division switch optical signals carrying information using optical and opto-electronic means controlled by optical signals.